Accurate delivery of liquids, particularly in the field of infusion of medical liquids, and especially in the field of IV infusion of pharmaceuticals, is becoming more and more critical as the potency of infusible therapeutic agents continues to increase. Historically, IV infusions were performed with the active ingredient dissolved in an appropriate vehicle hanging in a bag from a pole above the patient. Such gravity bag infusion, while able to provide the medical liquid to the patient in a continuous fashion, suffered from accuracy problems. Because the driving force is derived from the distance the solution in the bag is above the infusion site, the driving force would change with any change in distance, such as when the bag empties of solution, when the patient moved from a prone position to lying on a side, and especially when the patient arose from bed.
The rate of infusion would also change with temperature due to changes in the viscosity of the medical liquid, and could drastically change when different medical liquids were used. Further, the rate of infusion would change significantly when different infusion sets were used due to the manufacturing tolerances of the inside diameter of the flow tube. While the user of a gravity bag infusion set could adjust the infusion rate using a roller clamp on the tube and counting the drips per second in a drip chamber included in the infusion set, the accuracy of such an adjustment was qualitative at best.
To overcome these limitations, positive displacement pumps have replaced gravity bags for the infusion of medical liquids, especially for those liquids where precise control of the delivery rate is important. These positive displacement pumps include syringe pumps, where the volume of fluid to be infused is relatively small and peristaltic pumps where the volume of the medical liquid to be infused is relatively large. Positive displacement pumps are designed in such a way that a fixed volume of liquid is delivered independent of the pressure required to deliver the liquid or the viscosity of the delivered liquid. With these two variables removed, infusion pumps overcame two of the major obstacles to accurate delivery which are inherent in gravity bag infusion systems.
These positive displacement medical liquid infusion systems are “open-loop” systems in that they rely on the dimensional accuracy of system components to achieve accurate delivery. For syringe pumps, for example, the accuracy of delivery is mainly dependent on the inside diameter of the barrel of the syringe in use. Since the accuracy depends on the square of the inside diameter, it is important to manufacture these syringes with close tolerances. For peristaltic pumps, the accuracy of delivery depends on the inside diameter of the flow tube at the site where the peristaltic pumping occurs. Again, the delivery accuracy depends on the inside diameter of the flow tube to the second power, so manufacturing tolerances are relatively tight.
If system components are used that are outside of the specification range, such positive displacement systems will inaccurately deliver the medical liquid. In such an instance, there is no provision to correct or compensate for the out of range dimension. In an attempt to address this problem, closed loop medical liquid infusion systems have been described (see, e.g., Frank U.S. Pat. No. 5,211,626). This system measures the flow rate of the medical liquid along the flow channel and then adjusts the position of a proportional valve to achieve the desired flow rate. This closed loop system, however, fails to distinguish between changes in flow rate due to temperature, viscosity, or flow channel dimensions. Because the flow rate has differing exponential dependence for these flow rate variables, the calculated valve adjustment is usually incorrect, resulting in inaccurate infusion of the medical liquid.
Despite the transition from simple and inexpensive gravity bags, the complicated and expensive infusion pumps typically fail to achieve highly accurate delivery or to provide evidence that the desired drug delivery is occurring or has occurred (other than that the motor is operating or has operated). The actual flow rate remains unverified and uncompensated.